Low defect density silicon carbide

ABSTRACT

A low dislocation density silicon carbide (SiC) is provided as well as an apparatus and method for growing the same. The SiC crystal, grown using sublimation techniques, is preferably divided into two stages of growth. During the first stage of growth, the crystal grows in a normal direction while simultaneously expanding laterally. Although dislocations and other material defects may propagate within the axially grown material, defect propagation and generation in the laterally grown material are substantially reduced, if not altogether eliminated. After the crystal has expanded to the desired diameter, the second stage of growth begins in which lateral growth is suppressed and normal growth is enhanced. A substantially reduced defect density is maintained within the axially grown material that is based on the laterally grown first stage material.

CROSS-REFERENCES TO RELATED APPLICATIONS

[0001] This application claims priority of U.S. Provisional PatentApplication Serial No. 60/182,553, filed Feb. 15, 2000.

FIELD OF THE INVENTION

[0002] The present invention relates generally to silicon carbide and,more particularly, to a method and apparatus for growing low defectdensity silicon carbide.

BACKGROUND OF THE INVENTION

[0003] Silicon carbide (SiC) has a number of characteristics that makeit an ideal candidate for a variety of semiconductor applications,primarily those requiring high power handling capabilities. Arguably themost important characteristic of SiC is its indirect bandgap, resultingin relatively high recombination lifetimes and the ability to producehigher voltage junctions than those that can be produced from a directbandgap material. The large bandgap of this material also provides fornegligible current leakage up to 500° C., thereby allowing for hightemperature operation without excessive leakage current or thermalrunaway. The switching frequency of SiC devices is much higher than thatof a device fabricated from silicon or gallium arsenide due to SiC'shigh breakdown strength and the resultant reduction in minority carrierstorage and associated switching losses. Lastly, due to the highjunction temperature and the high thermal conductivity of SiC, devicesfabricated from SiC have reduced cooling requirements.

[0004] Although semiconductor devices based on SiC offer vastimprovements over devices fabricated from silicon, in order to realizethese improvements materials must be fabricated with much lower defectdensities than have been obtainable heretofore. As noted by the authorsin the 1999 article entitled SiC Power Devices, Naval Research Reviews,Vol. 51, No. 1 (1999), in order to scale up devices fabricated from SiC,the density of dislocations as well as the density of micropipes must bereduced. Conventional SiC material has a dislocation density between 10⁵and 10⁶ per square centimeter and a micropipe density between 10² and10³ per square centimeter. Some extremely high quality SiC material hasbeen grown with dislocation densities on the order of 10⁴ per squarecentimeter. Unfortunately, even this dislocation density is at least anorder of magnitude too high for many semiconductor applications. Id. atpage 21.

[0005] U.S. Pat. No. 5,679,153 discloses a technique of growing SiCepitaxial layers using liquid phase epitaxy in which the density ofmicropipes is substantially reduced or eliminated. In one aspect of thedisclosed technique, an epitaxial layer of SiC is formed on a bulksingle crystal of SiC, the epitaxial layer being of sufficient thicknessto close micropipe defects propagated from the bulk crystal. In order toform an electronically active region for device formation, a secondepitaxial layer is formed on the first epitaxial layer by chemical vapordeposition. Based on this technique, SiC layers having micropipedensities of between 0 and 50 micropipes per square centimeter on thesurface were claimed.

[0006] Although techniques have been disclosed to achieve SiC materialswith low micropipe densities, these techniques do not lend themselves togrowing bulk materials, i.e., materials that are at least a millimeterthick or more preferably, at least a centimeter thick. Additionally,these techniques do not impact the dislocation densities of thematerial. Accordingly, what is needed in the art is a technique ofgrowing bulk SiC material with defect densities on the order of 10³ persquare centimeter, more preferably on the order of 10² per squarecentimeter, and even more preferably on the order of 10 or lessdislocations per square centimeter. The present invention provides sucha technique and the resultant material.

SUMMARY OF THE INVENTION

[0007] In accordance with the invention, a low dislocation densitysilicon carbide (SiC) is provided as well as an apparatus and method forgrowing the same. The SiC crystal, grown using sublimation techniques,is divided into two stages of growth. During the first stage of growth,the crystal grows in a normal direction while simultaneously expandinglaterally. Preferably during this stage the ratio of the lateral growthrate to the axial growth rate is between 0.35 and 1.75. Althoughdislocations and other material defects may propagate within the axiallygrown material, defect propagation and generation in the laterally grownmaterial are substantially reduced, if not altogether eliminated. Afterthe crystal has expanded to the desired diameter, the second stage ofgrowth begins in which lateral growth is suppressed and normal growth isenhanced. Preferably during this stage the ratio of the lateral growthrate to the axial growth rate is between 0.01 and 0.3, and morepreferably between 0.1 and 0.3. A substantially reduced defect densityis maintained within the axially grown material that is based on thelaterally grown first stage material.

[0008] In one aspect of the invention, a SiC material is provided with alow defect density, defects including both dislocations and micropipes.The defect density in the grown SiC is less than 10⁴ per squarecentimeter, preferably less than 10³ per square centimeter, morepreferably less than 10² per square centimeter, and still morepreferably less than 10 per square centimeter. In at least oneembodiment, SiC is grown comprised of an axially grown region and alaterally grown region, the laterally grown region having the desiredlow defect density. In another embodiment of the invention, the SiC iscomprised of a central region having a first defect density and aperimeter region encircling the central region that has a second defectdensity. The second defect density is substantially less than the firstdefect density and is less than 10³ per square centimeter, preferablyless than 10² per square centimeter, and more preferably less than 10per square centimeter. In another embodiment of the invention, the SiCmaterial is comprised of a SiC seed crystal, a first crystalline growthregion initiating at a growth surface of the SiC seed crystal andfollowing an axial growth path, and a second crystalline growth regionof the desired defect density initiating at a growth surface of the SiCseed crystal and following a laterally expanding growth path. Thelaterally expanding growth path is at an angle of at least 25 degrees,and preferably at least 45 degrees, from the normal, i.e., axial, growthpath.

[0009] In another aspect of the invention, a method of growing a SiCmaterial with a low dislocation density is provided. In at least oneembodiment, a SiC seed crystal is introduced into a sublimation systemwherein both axial and lateral crystal growth is promoted, at leastduring one stage of growth. Propagation of dislocation defects,including micropipes, from the seed crystal into the laterally growncrystal is substantially reduced as is generation of dislocation defectswithin this region. In at least another embodiment of the invention, aSiC seed crystal is introduced into a sublimation system and heated to atemperature sufficient to cause sublimation. Temperature gradientswithin the sublimation system as well as temperature differentialsbetween the crystallization growth front and adjacent surfaces promote afirst stage of free space crystal expansion wherein the crystallizationfront expands both axially and laterally followed by a second stage offree space crystal expansion wherein the crystallization front expandsaxially while lateral expansion is suppressed.

[0010] In another aspect of the invention, an apparatus for use ingrowing a SiC material with a low dislocation density is provided. In atleast one embodiment of the invention, the apparatus includes a ringelement that promotes lateral crystal expansion, preferably through theuse of a conical surface. The ring element may also be used to shieldthe edge of the SiC seed from the growth process. The ring element mayalso include a second surface, preferably conical, that promotes lateralcrystal contraction. Preferably the ring element inner surfaces arecomprised of either Ta_(x)C_(y) or Nb_(x)C_(y). In at least oneembodiment of the invention, the apparatus also includes a graphite heatsink coupled to a non-growth surface of the SiC seed crystal, a growthchamber with inner surfaces preferably comprised of either Ta_(x)C_(y)or Nb_(x)C_(y), and means for applying temperature gradients to thecrucible.

[0011] A further understanding of the nature and advantages of thepresent invention may be realized by reference to the remaining portionsof the specification and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 schematically illustrates the regions of defect freecrystal growth in accordance with the invention;

[0013]FIG. 2 schematically illustrates a reduction in the core regionduring crystal growth;

[0014]FIG. 3 illustrates the basic methodology applied to achieve lowdefect density SiC;

[0015]FIG. 4 illustrates the preferred design of the sublimation system;

[0016]FIG. 5 illustrates a detailed cross-section of the critical growthregion of the preferred design of the invention;

[0017]FIG. 6 illustrates the calculated temperature distribution withinthe growth cell and in the growing crystal for a ring element cone angleof 45 degrees and a growth period of approximately 4 hours;

[0018]FIG. 7 illustrates the calculated temperature distribution withinthe growth cell and in the growing crystal for a ring element cone angleof 70 degrees and a growth period of approximately 4 hours;

[0019]FIG. 8 illustrates the calculated temperature distribution withinthe growth cell and in the growing crystal for a ring element cone angleof 45 degrees and a growth period of approximately 22 hours;

[0020]FIG. 9 illustrates the calculated temperature distribution withinthe growth cell and in the growing crystal for a ring element cone angleof 70 degrees and a growth period of approximately 22 hours;

[0021]FIG. 10 illustrates a one dimensional temperature distributionover the ring element cell wall and the crystal interface;

[0022]FIG. 11 illustrates the ring element wall locations and thecrystal interface locations that correspond to the data points shown inFIG. 10;

[0023]FIG. 12 illustrates the distribution of the principal component ofthe thermal elastic stress tensor as computed for the temperaturedistributions shown in FIG. 6; and

[0024]FIG. 13 illustrates the distribution of the principal component ofthe thermal elastic stress tensor as computed for the temperaturedistributions shown in FIG. 8.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

[0025] The dislocations in (0001) silicon carbide (SiC) seed crystalsare primarily threading and screw dislocations in the<0001> crystaldirection. Micropipe defects are basically screw dislocations with aBurger's vector that is so large that the core of the screw is empty.The inventors have found that by growing a crystal under the appropriateconditions in the radial direction (i.e., lateral direction) rather thanthe axial direction, the multiplication of<0001> dislocations issuppressed. Accordingly, under the appropriate conditions, a defect freeSiC crystal can be grown using sublimation techniques.

[0026] In the preferred embodiment of the invention, as illustrated inFIG. 1, crystal growth is divided into two stages. During the firststage of growth, the crystal grows in a normal direction (i.e.,vertically) while simultaneously expanding laterally. Preferably duringthis stage the ratio of the lateral growth rate to the axial growth rateis between 0.35 and 1.75. After the crystal has expanded to the desireddiameter, lateral growth is suppressed while normal growth is enhanced(i.e., the second stage of growth). Preferably during this stage theratio of the lateral growth rate to the axial growth rate is between0.01 and 0.3, and more preferably between 0.1 and 0.3. In the preferredembodiment this ratio is not too low, thereby achieving a slightlyconvex growth surface and avoiding singularization of this surface.Lateral crystal growth is limited by the size of the crucible, which, inturn, is primarily limited by the ability to achieve the requiredtemperature gradients.

[0027] According to the invention, although dislocations and othermaterial defects propagate within core region 101, defect propagationand generation is substantially reduced, if not altogether eliminated,in the laterally grown area 103. Additionally, a substantially reduceddefect density is maintained within the axially grown material that isbased on the laterally grown material, i.e., regions 105. Preferably thegrowth conditions are chosen to reduce the size of core region 101during growth as illustrated in FIG. 2.

[0028] In order to achieve defect free lateral crystal growth, theinventors have found that preferably a number of conditions are met.These conditions, discussed in further detail below, include:

[0029] (i) High quality seed crystal—Preferably the growth surface ofthe seed crystal is defect free, thereby minimizing the propagation ofdefects within the core region of the grown crystal.

[0030] (ii) Shielding the back surface of the seed crystal to preventinitiation of dislocations and other microscopic defects (e.g., planardefects) which can propagate through the crystal and impact the qualityof the grown crystal.

[0031] (iii) Selection of the proper growth angle—Generally angle 107 inFIG. 1 must be greater than 25 degrees, and preferably greater than 45degrees. This angle is primarily determined by two factors; first, thevertical temperature gradient between the source and the seed crystaland second, the axial temperature gradient between the center of thecrucible and the crucible walls.

[0032] (iv) Selection of an appropriate seed crystal diameter—Generallythe diameter of the seed crystal should be less than 30 percent of thediameter of the crystal to be grown.

[0033] (v) Prevention of polycrystalline growth—Requires the eliminationof contact between the laterally growing material and the crucibleside-walls thus insuring free-space expansion of the crystal. This isachieved by maintaining a temperature differential between the laterallygrowing crystal and the crucible side-walls.

[0034]FIG. 3 illustrates the basic methodology applied to achieve defectfree SiC while corresponding FIG. 4 illustrates the preferred design forthe furnace and crucible. Initially a SiC seed crystal 401 is selectedand prepared. (Step 301) Seed crystal 401 can be grown using any of anumber of well known techniques (e.g., Lely method). In the preferredembodiment a seed crystal with minimal defects is used, typically on theorder of 10⁵ per square centimeter or less. Also preferably the seedcrystal has minimal, if any, micropipe defects. The ratio of thediameter of seed crystal 401 to the diameter of the crystal to be grownis preferably less than 0.3.

[0035] In the preferred embodiment of the invention, surface mechanicaldefects are removed from the surface of the seed crystal usingconventional surface preparation techniques, e.g., grinding, polishing,and chemical etching. In this embodiment, approximately 50 microns isremoved although the removal of additional material in excess of a 50micron layer may be required in order to achieve the desired surface.Preferably the surface finish has an RMS roughness of 50 Angstroms orless.

[0036] During crystal growth, it is important to prevent graphitization.(Step 303) Accordingly, the back surface of seed crystal 401 ispreferably shielded during evaporation. The preferred method ofshielding the non-growth surfaces of seed crystal 401 is to place thegrowth surface of the crystal onto the flat, polished surface of atantalum disk. Then the wafer is annealed in vacuum for a few minutes at1700 to 1750° C., yielding a dense graphite layer 403 on those seedcrystal surfaces not in contact with the tantalum disk. Once formed,back surface graphite layer 403 is coupled to a holder 405. Preferablyholder 405 is comprised of graphite or pyrographite and a graphite basedglue is used to couple seed crystal 401 to holder 405, the glueeliminating voids in the joint between the two surfaces. In addition tohelping to prevent graphitization, the above-described crystal seatingprocess also prevents localized temperature non-uniformities in theseating area that arise from having voids between the back surface ofthe seed crystal and the crucible lid (e.g., the seed holder).

[0037] In an alternate embodiment, the dense graphite layer 403 is grownon all seed crystal surfaces and subsequently removed from the growthsurface, for example using an epi-polishing technique.

[0038] In order to shield the non-growth seed surfaces of the seedcrystal from evaporation, in the preferred embodiment of the inventionseed crystal 401 is sealed within a multi-element system that includes aring element 407. (Step 305) Ring element 407 is not only integral tothe sealing system, it also helps to shape the desired lateral growth ofthe crystal. The outer surface of element 407 is cylindrical while theinner surfaces are generally conical and coated with Ta_(x)C_(y) orNb_(x)C_(y). Preferably the inner surfaces of element 407 are comprisedof a pair of conical surfaces as shown in FIG. 4.

[0039] Seed crystal 401 is pressed and sealed to ring element 407 usinga gas impermeable graphite foil 409. As shown, the inner diameter of theportion of ring element 407 adjacent to seed crystal 401 as well as theinterposed gas impermeable foil 409 is smaller than the outer diameterof crystal 401. Accordingly, the edge of crystal 401 is unexposed, thuscontrolling growth of crystal defects that typically would originate atthe crystal edge, the crystal edge defined as the juncture of thecrystal face surface and the crystal side surface. In addition topreventing exposure of the crystal edge, ring element 407 provides ameans of achieving the desired lateral temperature gradient and thus, aspreviously noted, provides a means of controlling the lateral growth ofthe crystal.

[0040] In the preferred embodiment of the invention, the inner surfaceof ring element 407 is comprised of a pair of conical surfaces, the pairof conical surfaces providing a slight necking down of the crystalgrowth surface. As such, prior to undergoing lateral expansion, thecrystal growth surface undergoes an initial period of contraction. In analternate embodiment, ring element 407 is comprised of a single conicalinner surface that defines the laterally expanding crystal surface. Inan alternate embodiment, ring element 407 is comprised of a singleconical inner surface that defines the laterally expanding crystalsurface and a non-conical inner surface immediately adjacent to the seedcrystal that insures that the crystal growth interface initiallyundergoes a period of contraction prior to undergoing lateral expansion.As in the preferred embodiment, in the alternate embodiments the innerdiameter of the portion of element 407 in contact with foil 409 (andadjacent to crystal 401) is smaller than the outer diameter of seedcrystal 401, thus controlling growth of edge defects.

[0041] A second gas impermeable graphite foil 411 is used to seal theback surface of seed crystal 401 and holder 405 to a graphite heat sink413. Foil 411 helps to achieve a good thermal contact between crystal401/holder 405 and heat sink 413, the thermal contact preferably beingcontinuous across the entire heat sink interface. As shown, the outerdiameter of graphite heat sink 413 is substantially equal to the outerdiameter of ring element 407. The stack, comprised of heat sink 413,holder 405, seed crystal 401, ring element 407, and seals 409 and 411,is press fit within a thin-wall graphite cylinder 415 (Step 307),thereby preventing reactive gases such as Si, SiC, and/or SiC₂ fromreaching the non-growth surfaces of seed crystal 401.

[0042] A source 417 is placed within a growth chamber 419. (Step 309).Seed crystal 401, the multi-element sealing system and graphite cylinder415 are then located within the growth chamber. (Step 311) Growthchamber 419 is fabricated from Ta_(x)C_(y), Nb_(x)C_(y), or graphite. Ifgraphite is used for growth chamber 419, the inner surfaces of thechamber are coated with either Ta_(x)C_(y) or Nb_(x)C_(y). Preferablythe distance between the seed crystal growth surface and the sourcematerial 417 is less than 30 percent of the diameter of source 417, thusallowing quasi-equilibrium vapor phase conditions to be maintained.

[0043] Source 417 is fabricated in such a manner as to suppress theformation of source particles during crystal growth. In the preferredembodiment of the invention, this goal is achieved by annealingelectronic grade SiC powder or a mixture of Si and C powders at atemperature of between about 2100 and 2500° C. for approximately 1 hour.As a result of the annealing process, a dense deposit is formed thateliminates particle formation during crystal growth. In order to obtaindoped SiC crystals using the invention, the desired dopants and/orimpurities (e.g., nitrogen, boron, aluminum, indium, vanadium,molybdenum, scandium, chromium, iron, magnesium, tin, and zirconium) areincluded in source 417.

[0044] Growth chamber 419 is placed within a two-piece graphite crucible421 (step 313), the shape (e.g., the tapered portions) of which isdesigned to provide the temperature gradients described in fartherdetail below. In order to prevent graphitization, a requirement of thepresent invention, the stoichiometry of the vapor within the growthchamber, i.e., the ratio of silicon to carbon, must remain relativelyconstant during crystal growth. One method of realizing this objectiveis to minimize material losses. Accordingly, in the preferred embodimentof the invention, the rate of material loss during the growth process ismaintained at a level of less than 0.5 percent of the initial sourceweight per hour. In particular this rate loss is achieved by firstlocating graphite crucible 421 within a high temperature furnace 423,preferably an RF induction furnace as shown. (Step 315) Graphite foam425 is used to suppress heat losses from the furnace. The furnace, alongwith the growth chamber, is next evacuated down to a pressure of 10⁻⁵torr or less (step 317) and then heated to a temperature ofapproximately 1500° C. (step 319). Chamber 419 is then sealed,preferably using different types of graphite with different coefficientsof thermal expansion, in order to prevent graphitization. (Step 321)

[0045] After chamber sealing, the furnace is filled with pure argon orargon with traces of nitrogen. (Step 323) To obtain the desiredresistivity within the grown crystal, the partial pressure of the gasfilled furnace is maintained within a range of 10⁻¹ to 10⁻⁴ torr.Crucible 421 and chamber 419 are then heated to a temperature of between1900 and 2400° C. at a rate of between 6 and 20° C. per minute. (Step325)

[0046] During crystal growth, crucible 421 is axially rotated at a rateof approximately 1 to 5 revolutions per minute. (Step 327) As thecrystal grows, the required temperature gradients are achieved, at leastin part, by altering the relative positions of crucible 421 and furnace423. (Step 329) Typically the rate of movement is approximatelyequivalent to the rate of crystal growth, i.e., between 0.1 and 1.5millimeters per hour.

[0047] Preferred Crystal Growth Methodology

[0048] In addition to the method and apparatus described above, theinventors have found that certain growth methodologies are preferred. Aspreviously noted, free-space expansion of the crystal during growth iscritical to achieving defect free SiC. Accordingly, it is important toprevent the formation of polycrystalline deposits on all surfaces thatsurround seed 401, such surfaces including ring element 407, seed holder405, graphite cylinder 415, and growth chamber 419. Insuring that thetemperature of the surface in question is higher than that of the seedis the preferred technique for preventing polycrystalline deposits. Atthe same time, however, it is important that the temperaturedifferential between the seed and the adjacent surfaces not be toogreat, otherwise lateral crystal growth may be deterred. Accordingly,the inventors have found that the temperature differential between thecrystallization front and the adjacent surface before thecrystallization front should be in the range of 1 to 5° C.

[0049] The inventors have found that during the period of time in whichthe crystal is undergoing lateral expansion, a temperature drop ofbetween 5 and 25° C., and preferably between 5 and 10° C., should bemaintained between seed 401 and source 417. This temperaturedifferential aids in the suppression of normal (i.e., non-lateral)crystal growth. Preferably there is a lateral dependence to thetemperature differential such that the smallest temperature differentialoccurs at the center of the seed, increasing with lateral distance. As aresult, a convex crystallization growth front is formed which aids inthe elimination of micropipe propagation.

[0050] As previously described, preferably the angle between the normalcrystal growth and the lateral crystal growth (e.g., angle 107 ofFIG. 1) is greater than 25 degrees. If the angle is less than 25degrees, the defects of seed crystal 401 and any defects that may begenerated during the initial crystal growth will continue to propagatethroughout the newly grown crystal. If the angle is greater than 45degrees, as in the preferred embodiment of the invention, typically allof the defects will move towards the lateral surface and, once thelateral surface is reached, not participate further in the growthprocess. In those cases in which not all of the defects are eliminatedfrom participation in the growth process, the defect density in thelaterally grown material is typically on the order of 10² per squarecentimeter or less, and more typically on the order of 10 per squarecentimeter or less. In the intermediate situation in which the angle isbetween 25 and 45degrees, the expansion of defects into the laterallygrowing crystal body is typically observed. If seed crystal 401 is of ahigh quality, however, angles within this range may yield crystals ofsufficiently low defect density.

[0051] Initially, lateral crystal growth dominates (step 331), thelaterally grown crystal being free of micropipes and having a defect(e.g., dislocations, micropipes) density less than 10⁴ per squarecentimeter, preferably less than 10³ per square centimeter, morepreferably less than 10² per square centimeter, still more preferablyless than 10 per square centimeter, and still more preferably with zerodefects per square centimeter. As observed, this material is free of anygraphite inclusions. Crystal growth, dominated by lateral crystalgrowth, continues until the desired crystal diameter is reached, thiscrystal diameter being defined by the growth chamber in general, and forthe embodiment illustrated in FIG. 5, by ring element 407. Once thecrystal reaches the desired diameter (step 333), the verticaltemperature gradient is changed to promote normal, i.e., vertical,crystal growth (step 335). In order to achieve the desired change in thetemperature gradient, the relative positions of furnace 423 and crucible421 are changed. In the preferred embodiment of the invention, furnace423 is an inductive furnace and the coils of the furnace are movedrelative to crucible 421. Alternately, or in addition to changing therelative positions of the furnace and the crucible, the temperaturewithin portions of the furnace may be changed. Preferably the axialtemperature gradient, i.e., the gradient between the source and thegrowth surface, is in the range of 10 to 50° C. per centimeter, yieldingthe desired normal growth rate of between 0.4 and 1.5 millimeters perhour.

[0052] During the last stage of crystal growth it is important toprevent considerable lateral crystal expansion. It is also important, aspreviously described, to prevent SiC deposits from forming on thecrucible side walls (e.g., the side walls of ring element 407, graphitecylinder 415, and growth chamber 419). Accordingly, a higher side walltemperature is maintained relative to the temperature of the seedcrystal, preferably the temperature difference being at least 10° C.,more preferably between 10 and 30° C., and still more preferably between10 and 15° C. The higher side wall temperature radiatively heats thesides of the growing crystal, thereby achieving hotter crystal sidewalls than the normal growth surface of the crystal. As a result, all ofthe vapor species are consumed at the normal growth surface of thecrystal and growth on the crucible side walls is suppressed.Additionally, this temperature difference insures that the growingcrystal does not come into contact with the crucible side walls, suchcontact being a major source of defects.

[0053] The inventors have also found that the temperature gradient inthe growing crystal must be maintained at a relatively low number,preferably on the order of 5° C. per centimeter or less. If thetemperature gradient becomes too large, strain is created within thegrowing crystal, resulting in the formation of dislocations or otherdefects.

[0054] Detailed Growth Region

[0055]FIG. 5 is a cross-sectional view of the critical growth region inthe preferred embodiment of the invention. In this embodiment the SiCseed crystal 501 is held within a portion of ring element 503. Agraphite foil ring 505 is interposed between ring element 503 and thegrowth surface of crystal 501, foil ring 505 sealing the seed crystal tothe ring element. The side and back surfaces of crystal 501 are coveredwith a graphite foil 507. A graphite disk 509 is coupled to seed crystal501 via graphite foil 507. The primary purpose of disk 509 andinterposed graphite foil 507 is to aid in the removal of heat fromcrystal 501. Additionally, disk 509 provides a support surface forcrystal 501 as well as a means for conveniently applying pressure to thecrystal with graphite ring 511, thereby achieving a seal between thecrystal and element 503. Graphite foils 505 and 507 are typicallybetween 0.25 and 0.80 millimeters thick.

[0056] Ring element 503 is preferably press fit within a graphitecylinder 513. Graphite foil 515, typically between 0.25 and 0.80millimeters thick, is preferably interposed between the outer wall ofring element 503 and the inner wall of cylinder 513, thus helping toachieve a good pressure and thermal seal. Graphite foam 517 is used tosuppress heat losses from the furnace.

[0057] In this embodiment of the invention, ring element 503 isfabricated from graphite with inner surfaces 519 coated with Ta_(x)C_(y)or Nb_(x)C_(y). Diameter D, the largest inner diameter of element 503,is 30 millimeters although there are no major limitations to increasingthis diameter, thereby yielding a larger grown crystal. Diameter d, thesmallest inner diameter of element 503 is selected such that the ratioD/d is greater than 3. Angle 521 is selected, as previously disclosed,to be greater than 25 degrees and preferably less than 90 degrees.

[0058] Thermal Analysis

[0059] FIGS. 6-10 provide calculated temperature distributions withinthe growth cell and in the growing crystal for a specific embodiment ofthe invention. In FIGS. 6-9 the seed crystal is indicated as substrate601, the crystal growth interface is indicated as surface 603, and thering element is indicated as element 605. For purposes of this analysis,ring element 605 is comprised of a single conical surface rather than apair of conical surfaces as shown in the ring elements of FIGS. 4 and 5.

[0060]FIGS. 6 and 7 illustrate the temperature distribution afterapproximately 4 hours of growth while FIGS. 8 and 9 illustrate thetemperature distribution after approximately 22 hours of growth. Thecone angle for the ring element in FIGS. 6 and 8 is 45 degrees while thecone angle for the ring element in FIGS. 7 and 9 is 70 degrees.

[0061]FIG. 10 illustrates a one dimensional temperature distributionover the ring element cell wall, i.e., line 1001, and the crystalinterface, i.e., line 1003. The ring element wall locations and thecrystal interface locations that correspond to the data points shown onlines 1001 and 1003, respectively, are shown in FIG. 11.

[0062] Thermal Elastic Stress Distribution

[0063] As disclosed above, the temperature gradient within the growingcrystal must be maintained at a relatively low number, preferably 5° C.per centimeter or less. FIGS. 12 and 13 illustrate the distribution ofthe principal component of the thermal elastic stress tensor σ_(rz)given in Pascals, as computed for the temperature distributions shown inFIGS. 6 and 8. The illustrated thermal elastic stress component isresponsible for gliding of dislocations.

[0064] The computed results indicate that the σ_(rz) value does notexceed the SiC plasticity threshold in the major portion of the crystal.Accordingly, the probability of generating dislocations within thegrowing crystal is negligible.

[0065] As will be understood by those familiar with the art, the presentinvention may be embodied in other specific forms without departing fromthe spirit or essential characteristics thereof. Accordingly, thedisclosures and descriptions herein are intended to be illustrative, butnot limiting, of the scope of the invention which is set forth in thefollowing claims.

What is claimed is:
 1. A silicon carbide material comprising a single crystal bulk silicon carbide substrate with a density of defects of less than 10⁴ per square centimeter, wherein said defects are comprised of micropipes and dislocations.
 2. The silicon carbide material of claim 1, wherein said density of defects is less than 10³ per square centimeter.
 3. The silicon carbide material of claim 1, wherein said density of defects is less than 10² per square centimeter.
 4. The silicon carbide material of claim 1, wherein said density of defects is less than 10 per square centimeter.
 5. The silicon carbide material of claim 1, wherein said single crystal bulk silicon carbide substrate is at least 1 millimeter thick.
 6. The silicon carbide material of claim 1, wherein said single crystal bulk silicon carbide substrate is at least 1 centimeter thick.
 7. A silicon carbide material, comprising: a single crystal silicon carbide seed crystal, said single crystal silicon carbide seed crystal having a first density of defects, said defects comprised of micropipes and dislocations; an axial region of re-crystallized silicon carbide, said axial region grown off of said single crystal silicon carbide seed crystal, said axial region having a second density of defects, said defects comprised of micropipes and dislocations; and a lateral region of re-crystallized silicon carbide, said lateral region grown off of said single crystal silicon carbide seed crystal, said lateral region having a third density of defects, said defects comprised of micropipes and dislocations, wherein said third defect density is less than said first defect density and less than said second defect density, and wherein said third defect density is less than 10⁴ per square centimeter.
 8. The silicon carbide material of claim 7, wherein said third density of defects is less than 10³ per square centimeter.
 9. The silicon carbide material of claim 7, wherein said third density of defects is less than 10² per square centimeter.
 10. The silicon carbide material of claim 7, wherein said third density of defects is less than 10 per square centimeter.
 11. The silicon carbide material of claim 7, wherein said axial region of re-crystallized silicon carbide has a first thickness and said lateral region of re-crystallized silicon carbide has a second thickness substantially equivalent to said first thickness, and wherein said first thickness is at least 1 millimeter thick.
 12. The silicon carbide material of claim 7, wherein said axial region of re-crystallized silicon carbide has a first thickness and said lateral region of re-crystallized silicon carbide has a second thickness substantially equivalent to said first thickness, and wherein said first thickness is at least 1 centimeter thick.
 13. A silicon carbide material comprising a single crystal silicon carbide crystal with a first region and a second region, wherein said first region is centrally located within said single crystal silicon carbide crystal and has a first density of defects and wherein said second region encircles said axially located first region and has a second density of defects, said defects comprised of micropipes and dislocations, wherein said second density of defects is less than said first density of defects, and wherein said second density of defects is less than 10³ per square centimeter.
 14. The silicon carbide material of claim 13, wherein said first density of defects is greater than 10⁴ per square centimeter.
 15. The silicon carbide material of claim 13, wherein said second density of defects is less than 10² per square centimeter.
 16. The silicon carbide material of claim 13, wherein said second density of defects is less than 10 per square centimeter.
 17. The silicon carbide material of claim 13, wherein said single crystal silicon carbide crystal has a thickness at least 1 millimeter thick.
 18. The silicon carbide material of claim 13, wherein said single crystal silicon carbide crystal has a thickness at least 1 centimeter thick.
 19. A silicon carbide material, comprising: a single crystal silicon carbide seed crystal, said single crystal silicon carbide seed crystal having a first density of defects, said defects comprised of micropipes and dislocations, said single crystal silicon carbide seed crystal having a growth surface; a first region of re-crystallized silicon carbide, said first region of re-crystallized silicon carbide initiating at said growth surface of said single crystal silicon carbide seed crystal, wherein a first portion of a crystallization growth front corresponding to said first region of re-crystallized silicon carbide follows an axial growth path, said first region of re-crystallized silicon carbide having a second density of defects, said defects comprised of micropipes and dislocations; and a second region of re-crystallized silicon carbide, said second region of re-crystallized silicon carbide initiating at said growth surface of said single crystal silicon carbide seed crystal, wherein a second portion of said crystallization growth front corresponding to said second region of re-crystallized silicon carbide follows a laterally expanding growth path, wherein an outermost edge of said second portion of said crystallization growth front is at an angle of greater than 25 degrees as measured from a normal growth axis, said second region of re-crystallized silicon carbide having a third density of defects, said defects comprised of micropipes and dislocations, wherein said third density of defects is less than said first density of defects and less than said second density of defects, and wherein said third density of defects is less than 10 per square centimeter.
 20. The silicon carbide material of claim 19, wherein said second density of defects is greater than 10⁵ per square centimeter.
 21. The silicon carbide material of claim 19, wherein said third density of defects is less than 10³ per square centimeter.
 22. The silicon carbide material of claim 19, wherein said third density of defects is less than 10² per square centimeter.
 23. The silicon carbide material of claim 19, wherein said third density of defects is less than 10 per square centimeter.
 24. The silicon carbide material of claim 19, wherein said first and second regions of re-crystallized silicon carbide are at least 1 millimeter thick.
 25. The silicon carbide material of claim 19, wherein said first and second regions of re-crystallized silicon carbide are at least 1 centimeter thick.
 26. The silicon carbide material of claim 19, wherein said angle is greater than 45 degrees. 